1. Field of the Invention
The invention relates to communication systems, and more particularly to a cellular mobile-radio system.
2. Description of the Prior Art
A cellular mobile-radio system of this type is known from "ECR900 Cellular Radiotelephone", M. Ballard et al, Electrical Communication, Vol. 63, No. 1, 1989, pp. 45-51, or from "Intelligente Netze beschleunigen Einfuhrung neuer Dienste", H. Auspurg, Telcom Report 12 (1989), Vol. 5, pp. 142-145. This involves the cellular mobile-radio system standardized throughout Europe as the so-called GSM System.
In this system, the source supplying the information signal that is to be transmitted by the radio station is the so-called "mobile communication switching center", and the radio station distant therefrom in space is the so-called base station. In the information signal source, i.e., in the mobile communication switching center, there is located a transmission facility which is connected by a line with the base station and which transmits the information signal originating from the information signal source to the base station. The base station, more precisely the radio station of the base station, sends out a microwave radio signal which has a specific carrier frequency and which contains the information signal.
In the known systems, the information signal is transmitted from the information signal source to the base station in the base band state, e.g., as a PCM 30 signal. The base station has carrier frequency facilities, which transform the information signal to be sent out from the base band state into the radio signal, so that it can be sent out through the radio connection to the mobile subscribers.
For the radio signal that a base station receives from the mobile subscribers, the base station in the known systems has RF demodulation facilities, generally also called RF carrier frequency facilities. These facilities recover from the received radio signal the information signal contained therein, e.g., a digital signal with a bit sequence frequency of approximately 8 Mbit/s, which is then transmitted in the base band state to the mobile communication switching center.
If the radio traffic to be carried out is so dense that the cells supplied by each base station are appropriately selected to be very small, i.e., a transition to so-called "microcells" is made, then this requires a very large number of cells and, thus, a very large number of base stations in order to supply a specific geographic region. If the known system is used as a basis for this, this means high costs, because a very large number of base stations must be present, and each of these is expensive primarily because of its RF carrier frequency facilities.